1. Field of the Invention
The present invention relates to an abrasive tape used for abrading and polishing the surface of a magnetic disk used, for example, in a hard disk device, a method for producing an abrasive tape, and a method for varnishing.
Priority is claimed on Japanese Patent Application No. 2008-172597 filed on Jul. 1, 2008 and the contents of which are hereby incorporated into this application.
2. Description of the Related Art
The recording density of magnetic disks used in hard disk devices is getting higher and the heads which float and travel on the magnetic recording surface are floating lower. In order to cope with these trends, it is required that the surface of a magnetic disk has a high flatness. Therefore, a process for producing a magnetic disk includes forming a magnetic layer or a protective layer on a non-magnetic substrate and after that, performing a varnishing process in which the surface of the layer is abraded using an abrasive tape to remove projections formed on or attached to the surface of the layer.
Such a varnishing process is carried out, for example, using an abrasive tape on which alumina abrasive grains is applied and includes abrading the surface of a medium by pressing the surface of a medium with the abrasive tape using a rubber contact roll. When such process is carried out, abnormal projections and the like on the surface of the medium are removed. Therefore, it is possible to allow a magnetic head to float quite low in a hard disk device and the like in which a magnetic disk is used (for example, refer to JP-A-11-277339).
As an abrasive tape (varnish tape) used in a varnishing process, a tape made of a polyester base film on which an abrasive layer is formed is generally used. When the abrasive layer comes into contact with the surface of a magnetic layer side of a magnetic disk and slides thereon, tiny dust particles adhering to the surface of the magnetic disk are removed and also abnormal projections and the like existing on the surface thereof are abraded and removed, thereby smoothing the surface. As an abrasive, a chromium oxide, α-alumina, a silicon carbide, a non-magnetic iron oxide, diamond, γ-alumina, α,γ-alumina, molten alumina, corundum, man-made diamond, and the like having an average particle size of around 0.05 μm to 50 μm are used (for example, refer to JP-A-9-054943).
Specifically, this kind of process for varnishing a magnetic disk using an abrasive tape is carried out by pressing the abrasive grain surface of the abrasive tape on the surface of a magnetic layer side of a magnetic disk while the magnetic disk is spinning. As a result, projections on the surface of the magnetic disk are removed and the surface thereof is smoothed. Herein, the abrasive tape is hung between a supply roll and a wind roll and travels therebetween, and is gradually supplied from the supply roll and wound on the wind roll. While the abrasive tape travels from the supply roll to the wind roll, the opposite surface (reverse surface) from the abrasive grain surface of the abrasive tape is pressed by a rubber backing roll, a felt, or the like to press the abrasive surface of the abrasive tape on the surface of the magnetic disk.
At present, the recording density of magnetic disks is getting higher and accordingly, it is necessary that the distance between the magnetic head and the magnetic disk becomes narrow. Therefore, contamination of the surface of the magnetic disk during the varnishing process is problematic.
According to our examinations, it has been found that alumina particles are included in materials contaminating the surface of the magnetic disk and that the alumina particles are abrasive particles detached from an abrasive tape during the varnishing process, shattered abrasive particles, or materials detached from the shattered particles. That is, it has been found that when projections on the surface of a magnetic disk are abraded and removed by the varnishing process, abrasive particles adhered to an abrasive tape are detached therefrom, the surface of the abrasive particles is slightly shattered (cleaved), the detached particles and powders of the shattered abrasive particles are adhered to the surface of the magnetic disk, thereby contaminating the surface of the magnetic disk.
Particularly in recent years, particles obtained by precipitation (crystal growth particles) rather than by shattering have been largely used as abrasive particles used in an abrasive tape. This is to decrease variation of the particle size and the surface shape of the abrasive particles in order to prevent producing even a tiny scratch on the surface to be subjected to the varnishing process as processing accuracy required in the varnishing process becomes higher. However, since the surface of the precipitation particles is smooth and has an almost spherical shape as compared to that of the shattered particles, it is difficult to stably hold the particles on the abrasive tape support. For this reason, it is considered that abrasive particles readily detach from an abrasive tape when the surface of a magnetic disk is subjected to a varnishing process.
In consideration of the above-mentioned problems, an advantage of some aspects of the present invention is to provide an abrasive tape capable of suppressing contamination of a magnetic disk by shattered abrasive particles and smoothing the surface of the magnetic disk; a method for producing an abrasive tape; and a varnishing process.
The present inventors have performed keen examination to solve the above-mentioned problems and as a result, they have found that when the surface of an abrasive particle layer of an abrasive tape used in a process for varnishing a magnetic disk is covered with a solid resin layer (coating layer) having the thickness in the range of 0.5 μm to 10 μm and the surface of the resin layer is covered with a liquid lubricant layer, it is possible to suppress the shattering of abrasive particles in the abrasive particle layer and the detachment of such shattered particles, and to decrease the contamination of the surface of the magnetic disk due to the abrasive tape, thereby completing the invention.